1. Field of the Invention
The present invention relates to a transflective liquid crystal display (LCD) device. And more particularly, it relates to an array substrate for use in a transflective LCD, which is used in a transmissive mode or/and a reflective mode.
2. Description of the Related Art
The cathode-ray tube (CRT) was developed and is mainly used for display systems. However, flat panel displays are beginning to be incorporated into display systems because of their small dimension, low weight and low voltage power consumption. Presently, thin film transistor-liquid crystal displays (TFT-LCD) having a high resolution are being developed.
In general, LCD devices have various advantages in that, for example, they are relatively thin and require low power for operation, when compared to CRT display devices. Therefore, such LCD devices are good candidates to replace CRT display devices and have been a matter of great interest in a variety of technical fields.
The LCD devices are generally divided into three types: transmissive LCD devices; reflective LCD devices; and transflective LCD devices. The transmissive LCD device utilizes a back light device as a light source, while the reflective LCD device utilizes ambient light instead of a back light device. The transflective LCD device has both transmissive and reflective modes.
The transflective liquid crystal display (LCD) device selectively acts as the transmissive LCD device and as the reflective LCD device. Due to the fact that a transflective LCD device can make use of both internal and external light sources, it can operate in bright ambient light utilizing low power consumption.
FIG. 1 shows a typical transflective liquid crystal display (LCD) device 11. The transflective LCD device 11 includes upper and lower substrates 15 and 21 with a liquid crystal 23 interposed therebetween. The upper 15 and lower 21 substrates are sometimes referred to as a color filter substrate and an array substrate, respectively.
On a surface facing the lower substrate 21, the upper substrate 15 includes a black matrix 16 and a color filter layer 17. The color filter layer 17 includes a matrix array of red (R), green (G), and blue (B) color filters that are formed such that each color filter is bordered by the black matrix 16. The upper substrate 15 also includes a common electrode 13 positioned beneath the color filter layer 17 and the black matrix 16.
On a surface facing the upper substrate 15, the lower substrate 21 includes an array of thin film transistors (one of which being designated “T” in FIG. 1) that individually act as switching devices. The array of thin film transistors is formed to correspond with the matrix of color filters. A plurality of crossing gate lines 25 and data lines 27 are positioned such that a TFT is located near each crossing of the gate lines 25 and data lines 27. The lower substrate 21 also includes a plurality of pixel electrodes 18, each in an area defined between the gate lines 25 and the data lines 27. Such areas are often referred to as pixel regions “P.”
Each pixel electrode 18 includes a transmitting portion “A” and a reflective portion “C”. The transmitting portion “A” is usually formed from a transparent conductive material having good light transmissivity, for example, indium-tin-oxide (ITO) or indium-zinc-oxide (IZO). Alternatively, the transparent portion “A” can be a hole. Moreover, a conductive metallic material having a superior light reflectivity is used for the reflective portion “C”.
FIG. 2 is a schematic cross-section of one pixel of the typical transflective LCD device 11 to facilitate an understanding of the operation of such devices. As shown in FIG. 2, the transflective LCD device 11 includes lower 21 and upper 15 substrates and a liquid crystal layer 23 interposed therebetween. The upper substrate 15 includes a common electrode 13. However, in FIG. 2 the color filter layer 17 and black matrix 16 (see FIG. 1) are not depicted because FIG. 2 shows just one pixel of the typical transflective LCD device, and the color filter layer 17 does not affect the polarization state of light. The lower substrate 21 includes a transparent electrode 18a formed thereupon. On the transparent electrode 18a, an insulation layer 14 is formed. A reflective electrode 18b having the transmitting portion “A”, i.e., the hole, is formed on the insulation layer 14. The transparent 18a and reflective 18b electrodes function together as the pixel electrode 18. The transflective LCD device 11 also includes a backlight device 41.
The transflective LCD device 11 described above uses artificial light “F” generated from the backlight device 41 when operating in the transmissive mode, while the transflective LCD device 11 uses ambient light “D” from surroundings when operating in the reflective mode. Namely, as shown in FIG. 2, the transparent electrode 18a transmits light “F” irradiated from the backlight device 41, while the reflective electrode 18b reflects the ambient light “D.”
The transflective LCD device 11 is operable in both the reflective mode and the transmissive mode. In the reflective mode, the ambient light “D” passes through the upper substrate 15 and is reflected from the reflective electrode 18b back toward the upper substrate 15. With an electrical signal applied between the common electrode 13 and the pixel electrode (reflective electrode 18b and transparent electrode 18a) by the switching element “T” (see FIG. 1), the phase of the liquid crystal layer 23 changes. Thus, the light “D” passing through the liquid crystal layer 23 is colored by the color filter (see reference element 17 of FIG. 1) and is displayed as a colored pixel.
In the transmissive mode, the light “F” from the backlight device 41 passes through the transparent electrode 18a. With an electrical signal applied between the common electrode 13 and to the pixel electrode (reflective electrode 18b and transparent electrode 18a) by the switching element “T” (see FIG. 1), the alignment state of the liquid crystal layer 23 changes. Accordingly, the light “F” passing through the liquid crystal layer 23 is colored by the color filter 17 (see FIG. 1) and is displayed as a colored pixel.
As described above, since the transflective LCD device 11 has both transmissive and reflective modes, the transflective LCD device can be used anytime, day or night. It also has the advantage of being battery powered for an extended time due its low power consumption.
FIG. 3 is a plan view illustrating one pixel of an array substrate for a conventional transflective liquid crystal display device. As shown, gate lines 25 are arranged in a transverse direction, and data lines 27 are arranged in a longitudinal direction perpendicular to the gate lines 25. A thin film transistor (TFT) “T” is arranged at a crossover point of the gate lines 25 and the data lines 27. The TFT includes gate 61, source 63 and drain 65 electrodes. The gate electrode 61 extends from the gate line 25, and the source electrode 63 extends from the data line 27. The drain electrode 65 is spaced apart from the source electrode 63. A pixel electrode 19 is arranged on a region defined by the gate lines 25 and the data lines 27. The pixel electrode 19 includes a transparent pixel electrode 19a that is made of a transparent conductive material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO), and a reflective pixel electrode 19b that is made of a reflective conductive material such as aluminum (Al) or Al-alloy (for example, AlNb). The transparent pixel electrode 19a is usually formed under the reflective pixel electrode 19b and contacts the drain electrode 65 through a first contact hole 67. The reflective pixel electrode 19b that is usually formed over the transparent pixel electrode 19a is divided into a transmitting portion “A”, i.e., usually a hole, and a reflective portion “C.” Through a second contact hole 71, the reflective pixel electrode 19b contacts the transparent pixel electrode 19a, and is electrically connected with the drain electrode 65.
FIG. 4 is a cross sectional view taken along the lines IV-IV and V-V of FIG. 3, and illustrates layer elements of the array substrate for use in a conventional transflective LCD device. The manufacturing process of the array substrate of FIG. 3 will be briefly explained hereinafter.
In the first manufacturing step, the gate electrode 61 and the gate line (see reference element 25 of FIG. 3) are formed by depositing and patterning a first metal on a substrate 10. A gate insulation layer 26 is formed on the substrate 10 to cover the patterned first metal layer.
In the second manufacturing step, a semiconductor layer 64 is formed on the gate insulation layer 26, particularly over the gate electrode 61. The semiconductor layer 64 is comprised of an amorphous silicon layer (a-Si) and an impurity-doped amorphous silicon layer (n+/p+a-Si).
In the third manufacturing step, the source and drain electrodes 63 and 65 are formed on the semiconductor layer 64, and are made of the conductive metallic material selected from a group consisting of chromium (Cr), aluminum (Al), aluminum alloy (Al alloy), molybdenum (Mo), tantalum (Ta), tungsten (W), and antimony (Sb), and the like. By depositing and patterning these materials, not only are the source 63 and drain 65 electrodes formed, but the data line 27 is also formed on the gate insulation layer 26 such that the source electrode 63 is extends from the data line 27. The source 63 and drain 65 electrodes are spaced apart from each other and respectively overlap opposite ends of the gate electrode 61.
In the fourth manufacturing step, a first passivation layer 28 is then formed on and over the intermediates by depositing an organic or inorganic insulating material. After that, the first contact hole 67 that exposes a portion of the drain electrode 65 is formed by patterning the first passivation layer 28.
In the fifth manufacturing step, a transparent conductive material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO) is deposited on the first passivation layer 28 having the first contact hole 67 and then patterned to form the transparent pixel electrode 19a. Accordingly, the transparent pixel electrode 19a contacts the portion of the drain electrode 65 through the first contact hole 67.
In the sixth manufacturing step, a second passivation layer 69 is deposited on the first passivation layer 28 to cover the transparent pixel electrode 19a, and then patterned to form the second contact hole 71 over the first contact hole 67. The second contact hole 71 exposes a portion of the transparent pixel electrode 19a over the drain electrode 65. When performing the patterning process to form the second contact hole 71, a photo mask used for forming the second contact hole 71 is the same mask as used for forming the first contact hole 67.
In the seventh manufacturing step, the reflective pixel electrode 19b is formed by depositing and patterning a reflective conductive material such as aluminum (Al) or Al-alloy (for example, AlNB) on the second passivation layer 69 having the second contact hole 71. Accordingly, the reflective pixel electrode 19b contacts the transparent pixel electrode 19a through the second contact hole 71, thereby electrically interconnecting with the drain electrode 65. When patterning the reflective conductive material, the transmitting portion “A” (i.e., the hole) is formed at a nearly central portion of the pixel region.
However, the structure of the array substrate mentioned above has some problems and is limited. The reflective pixel electrode 19b is usually made of aluminum-based material because of its low resistance and high light reflectivity. However, from this reason, the aluminum-based material is easily oxidized and thus converts to aluminum oxide (Al2O3).
Accordingly, aluminum oxide (Al2O3) comprises an oxidized layer in an interface between the transparent pixel electrode 19a and the reflective pixel electrode 19b. Moreover, due to the aluminum oxide (Al2O3), the contact resistance in the interface between the transparent 19a and reflective 19b pixel electrodes becomes higher thereby deteriorating operating characteristics of the device.
To overcome this problem, another structure is proposed as shown in FIGS. 5 and 6. FIG. 5 is a plan view illustrating one pixel of an array substrate for another conventional transflective liquid crystal display device. Although FIG. 5 is similar to FIG. 3, the array substrate of FIG. 5 has a different structure from that of FIG. 3, and thus the differences between them are explained hereinafter.
As shown in FIG. 5, a pixel electrode 20 includes a transparent pixel electrode 20a made of a transparent conductive material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO), and a reflective pixel electrode 20b made of a reflective conductive material such as aluminum (Al) or Al-alloy (for example, AlNb), much as the pixel electrode 19 of FIG. 3. However, the transparent pixel electrode 20a is smaller in size than the reflective pixel electrode 20b, and has a protrusion that overlaps the drain electrode 65. This protrusion makes contact with the drain electrode 65 through a first contact hole 72. The transparent pixel electrode 20a is usually formed under the reflective pixel electrode and the reflective pixel electrode 20b is usually formed over the transparent pixel electrode 20a, much like the transparent and reflective pixel electrodes 19a and 19b of FIG. 3. Also, the reflective pixel electrode 20b is divided into a transmitting portion “A”, i.e., usually a hole, and a reflective portion “C”, which is much different from the reflective pixel electrode 19b of FIG. 3. The reflective electrode 20b of FIG. 5 contacts the drain electrode 65 through a second contact hole 73, and is electrically communicated with the drain electrode 65. Namely, the array substrate depicted in FIG. 5 has two differently positioned contact holes 72 and 73 in different places over the drain electrode 65. Thus, both the transparent pixel electrode 20a and the reflective pixel electrode 20b respectively contact the drain electrode 65.
FIG. 6 is a cross sectional view taken along the lines VI-VI and VII-VII of the LCD device of FIG. 5 and illustrates layer elements of the array substrate for use in another conventional transflective LCD device. The manufacturing process of the array substrate of FIG. 5 will be briefly explained hereinafter. However, since the manufacturing process is similar to that of array substrate of FIG. 4, some explanations for the manufacturing steps are omitted.
The first to fourth manufacturing steps of the array substrate depicted in FIG. 6 are basically the same as those of the array substrate depicted in FIG. 4. Therefore, the manufacturing process of the array substrate depicted in FIG. 6 will be explained beginning from the fifth manufacturing step.
In the fifth manufacturing step, a transparent conductive material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO) is deposited and then patterned on the first passivation layer 28 having the first contact hole 72 that is formed in an inner part of the drain electrode 65 during the fourth manufacturing step to form the transparent pixel electrode 20a. At this time, the transparent pixel electrode 20a has a protrusion that overlaps the drain electrode 65 as shown in FIG. 5. Thus, the protrusion of the transparent pixel electrode 20a contacts the portion of the drain electrode 65 through the first contact hole 72, and the transparent pixel electrode 20a is electrically communicated with the drain electrode 65.
In the sixth manufacturing step, the second passivation layer 69 is deposited on the first passivation layer 28 to cover the transparent pixel electrode 20a, and then patterned to form a second contact hole 73 in an outer part of the drain electrode 65. When forming the second contact hole 73, the first 28 and second 69 passivation layers are simultaneously etched. Thus, the second contact hole 73 penetrates the first 28 and second 69 passivation layers and exposes an outer portion of the drain electrode 65. In contrast to the patterning process of forming the second contact hole 73 in the array substrate depicted in FIG. 4, a photo mask used for forming the second contact hole 71 is not the same mask used for forming the first contact hole 72 because the first 72 and second 73 contact holes are formed in different locations above the drain electrode 65.
In the seventh manufacturing step, the reflective pixel electrode 20b is formed by depositing and patterning a reflective conductive material such as aluminum (Al) or Al-alloy (for example, AlNB) on the second passivation layer 69 having the second contact hole 73, much like the reflective pixel electrode 19b of FIG. 4. In the array substrate of FIG. 6, the reflective pixel electrode 20b, however, directly contacts the drain electrode 65 through the second contact hole 73, thereby being electrically connected with the drain electrode 65. When patterning the reflective conductive material, the transmitting portion “A” (i.e., the hole) is formed at a nearly central portion of the pixel region.
As described in FIGS. 5 and 6, the array substrate has two differently-placed contact holes 72 and 73 that connect the transparent pixel electrode 20a and reflective pixel electrode 20b to the drain electrode 65, respectively. As a result, the same photo mask can not be used for forming the first 72 and second 73 contact holes. Accordingly, an additional photo mask is required when forming the first or second contact hole 72 or 73.